Selective hydrogenation of acetylenes and dienes in a hydrocarbon stream

ABSTRACT

Acetylenes and dienes in a stream containing hydrogen, methane, C 2 -C 6  olefins and paraffins, C 2 -C 6  acetylenes and dienes, benzene, toluene, xylenes, and other C 6 + components are hydrogenated in a downflow boiling point reactor wherein the heat of reaction is absorbed by the liquid in the reactor which produces a vapor. Besides the feed to the reactor there is a recirculating stream which is fed at a rate sufficient to ensure that the catalyst particles within the reactor are wetted. A third stream, which is taken from a downstream distillation column, is fed to provide the make up mass corresponding to the mass evaporated in the reactor. The composition of the this third stream controls the steady state composition of the liquid flowing through the reactor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for selectivelyhydrogenating acetylenes and dienes in a hydrocarbon stream. Moreparticularly the invention relates to the selective hydrogenation ofacetylenes and dienes in a hydrocarbon stream containing hydrogen,olefins and smaller amounts of acetylenes and dienes using a downflowboiling point reactor.

[0003] 2. Related Information

[0004] The vapor product stream from the quench system of a hydrocarbonsteam cracker typically consists mainly of hydrogen, methane, C₂-C₆olefins and paraffins, C₂-C₆ acetylenes and dienes, benzene, toluene,xylenes, and other C₆+ components. Separation and recovery of theproducts according to carbon number is generally accomplished in asequential distillation system after the first separation of hydrogenfrom the methane in a high pressure cold box system. The design of thedistillation system is complicated by the fact that the differences inrelative volatility of the olefins, acetylenes, and dienes of the samecarbon number are small making it difficult to produce the pure olefinproducts. One method of circumventing this problem is to first separatethe carbon number fractions and then to selectively hydrotreat eachfraction to convert the acetylene and/or diene to its correspondingolefin or paraffin. This so called “back end” approach requires aseparate hydrotreating system for each carbon number fraction as well asthe addition of a requisite amount of hydrogen to each system. Analternative method is to hydrotreat the feed stream before separationusing the contained hydrogen as the source of hydrogen for theconversion. This so-called “front end” approach has the advantage ofremoving a significant portion of the hydrogen from the feed stream tothe cold box thereby reducing the size and refrigeration requirements ofthe cold box.

SUMMARY OF THE INVENTION

[0005] The present invention provides a “front end” hydrotreating systemthat allows for effective control of the temperature within a bed ofcatalyst which is hydrogenating acetylenes and dienes in a streamcontaining hydrogen, methane, C₂-C₆ olefins and paraffins, C₂-C₆acetylenes and dienes, benzene, toluene, xylenes, and other C₆+components. The invention utilizes a downflow boiling point reactorwherein the heat of reaction is absorbed by the liquid in the reactorwhich produces a vapor. Besides the feed to the reactor there is arecirculating stream which is fed at a rate sufficient to ensure thatthe catalyst particles within the reactor are wetted. A third stream,which is taken from a downstream distillation column, is fed to providethe make up mass corresponding to the mass evaporated in the reactor.The composition of the this third stream controls the steady statecomposition of the liquid flowing through the reactor. The compositionof this stream may be controlled by selecting the point from thedownstream distillation column from which the stream is drawn. The lowerthe draw point is in the column, the higher the boiling point of thecomponents in the third stream. The steady state composition of theliquid flowing through the reactor along with the pressure determinesthe reactor temperature profile.

[0006] In a “boiling point reactor” a liquid phase is always maintained,even if the reaction components would be vaporized by the exothermicheat of reaction. In any reaction where the reaction stream is likely tobe vaporized, an inert higher boiling component may be added to maintaina liquid phase.

BRIEF DESCRIPTION OF THE DRAWING

[0007]FIG. 1 is a flow diagram in schematic form of one embodiment ofthe invention.

[0008]FIG. 2 is graphical representation of the temperature profile in atypical reactor of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] Catalysts which are useful for the selective hydrogenation ofacetylenes and dienes include palladium oxide supported on alumina. Onesuch catalyst contains 0.34 wt. % palladium supported on ⅛ inch spheresdesignated G68C and supplied by Süd-Chemie (formerly United CatalystInc.). Another catalyst comprises 0.5 wt. % palladium supported on 8-12mesh spheres and designated E144SDU as supplied by Calcicat, Catalystand Performance Chemicals Division, Mallinckrodt, Inc. For best resultsthe catalyst is supported in structured packing as disclosed in commonlyowned U.S. Pat. No. 5,730,843. The catalyst may, however, be simplyloaded into the reactor.

[0010] Referring now to FIG. 1 selective hydrogenation of acetylenes anddiolefins in a hydrocarbon stream containing significantly largeramounts (molar basis) of hydrogen and olefins than the acetylenes anddiolefins is carried out in a downflow boiling point reactor. Thedownflow boiling point reactor, shown as column 10 is a verticallydisposed reactor containing the particulate catalyst supported in astructured packing at 12. The gaseous feed stream is fed via flow line101 to the top of the column 10. Also fed to the top of the reactor isliquid in flow line 104 which is a mixture of circulating stream in flowline 102 and stream in flow line 103 derived from distillation column 40as more particularly described below. Gas and liquid streams flowconcurrently downward through the column with the flow regime being gascontinuous. The concurrent flow of gas and liquid eliminates thepossibility of a runaway reaction.

[0011] The reactor 10 is operated adiabatically so that the heat ofreaction is accounted for by preferentially evaporating the lighterliquid phase components. Effluent from the reactor in flow line 105 isfed to vapor/liquid separator 20 where the vapor and liquid areseparated. The heat content of the vapor in flow line 106 includes theheat of reaction generated in the reactor 10 while its mass rate isequal to the combined flows of the streams in flow lines 101 and 103less slip stream 107 described below. Liquid in flow line 102 is fedback to the top of the reactor 10. The flow rate of the stream in flowline 102 is a variable and is maintained at least sufficient to ensurethat the catalyst particles are fully wetted at all positions in thereactor 10. The stream in flow line 103 provides make up masscorresponding to the mass evaporated in the reactor that leaves thereactor system as part of the stream in flow line 106. The compositionof the stream in flow line 103 controls the steady state composition ofliquid flowing through the reactor 10. This is an important operatingparameter that in combination with the reactor pressure determines thereactor temperature profile. A slip stream is taken by flow line 107 tocontrol the liquid inventory in the vapor/liquid separator vessel 20.

[0012] Column 40 is a C₅/C₆ splitter. Feed to the column is the vaporfrom the separator 20 in flow line 106. It is heated by indirect heatexchange in exchanger 30 with the recirculating stream in flow line 103.The column 40 is designed to recover a vapor distillate fraction viaflow line 108 which is essentially free of C₆+ components and a bottomsliquid product in flow line 109 which is essentially free of C₅ andlighter components. The overheads are taken via flow line 130 and passedthrough partial condenser 50 where the heavier components are condensed.The overheads are collected in receiver separator 60 where liquidhydrocarbon is withdrawn via flow line 120 and returned to the column 40as reflux. Water is taken out via flow line 110. As noted distillateproduct is removed via flow line 108.

[0013] The draw off position or tray of the recirculating stream in flowline 103 is an operating variable. Moving the take off point furtherdown the column increases the higher boiling components in the stream.Minimum operating pressure for the reactor 10 at a fixed temperatureprofile is achieved when the draw off is from the bottom stage of thecolumn 40.

EXAMPLE

[0014] Feed to the system depicted in FIG. 1 is the vapor product fromthe quench tower of an olefins producing steam cracker after compressionand acid gas (CO₂ and H₂S) removal. The reactor is loaded withapproximately 14,000 ft³ structured packing loaded with hydrogenationcatalyst. Bed dimensions are approximately 15 ft diameter by 70 ft long.Operating conditions for the reactor are: reactor top/bottom pressure250/240 psia; liquid recycle rate (stream in flow line 102) 4,000,000lbs./hr.; slip stream in flow line 107 2243 lbs./hr. The distillationcolumn 40 is a column configured with a 16.4 ft diameter 20 stage(theoretical) top section and 10.5 ft 20 stage (theoretical) bottomsection. Design conditions for the distillation column 40 are: refluxratio 0.18; reflux temperature 136° F., condenser pressure is 238 psia;column pressure drop is 2 psi; bottom stage side draw; decantertemperature 84° F. Heat and material balance results are given in TABLEI. Temperature profile across the reactor is given in FIG. 2. TABLE 1HEAT AND MATERIAL BALANCE 101 102 103 104 105 106 107 108 109 110Temperature F. 132 221.4 241.4 222.8 221.4 221.4 221.4 83.7 405.9 83.7Pressure psi 250 250 250 250 240 240 240 238 240 238 Vapor Frac 1 0 0 00.379 1 0 27,809.5 0 0 Mole Flow lbmol/hr 29,994.6 52453.1 3,537.955,991.0 84,546.9 32,064.4 29.4 757,208 578.6 167.8 Mass Flow lb/hr808,116.0 4,000,000 290,000 4,290,000 5,098,120 1,095,870 2,243 615,02047,885 3,022 Volume Flow 718,016.6 94,069 6,677 100,746 995,976 901,85353 −115.6 1,323 49 cuft/hr Enthalpy MMBtu/hr −37.8 −34.5 −1.4 −35.9−73.7 −39.2 0.0 6.0 −20.6 Mass Flow lb/hr 6,360 H2 9,260.1 119 0 1196,479 6,360 0 1,541 0 0 Co 1,540.9 58 0 58 1,599 1,541 0 0 0 Methane118,468.5 9286 0 9,286 127,755 118,463 5 118,468 0 0 Acetylen 4,280.8203 0 203 978 775 0 775 0 00 Ethylene 242,593.7 49,952 0 49,952 293,900243,920 28 243,948 0 0 Ethane 52,743.4 14,705 0 14,705 70,045 55,332 855,340 0 0 Meacetyl 5,139.0 666 0 666 1,410 744 0 744 0 0 Propadie5,197.5 2,583 0 2,583 5,743 3,158 1 3,160 0 0 Propylen 141,595.4 87,2810 87,281 233,196 145,866 49 145,915 0 0 Propane 4006.4 3,996 0 3,99610,556 6,558 2 6,560 0 0 Butadien 40,018.2 6,172 0 6,172 10,557 4,382 34,385 0 0 T-Butene 15317.0 23,503 0 23,503 38,820 15,304 13 15,317 0 01-Butene 15672.9 69,511 0 69,511 121,641 52,091 39 52,130 0 0 Cis2Bute15148.4 25,180 1 25,181 40,330 15,136 14 15,149 0 0 Isobuten 15705.220,525 0 20,525 36,230 15694 12 15,705 0 0 Isobutan 6571.8 7,591 0 7,59114,163 6,568 4 6,572 0 0 Butane 6,368.8 10,212 0 10,212 17,104 6,886 66,892 0 0 1Pentene 37318.5 140,912 2,356 143,268 190,449 49,457 7946,978 203 0 Hexane 10179.2 471,367 64,831 536,198 546,377 74,746 2641,669 8,509 0 Octane 1895.8 230,387 6,998 237,386 239,281 8,764 129 01,895 0 Benzene 27,486.7 1,826,330 167,100 1,993,430 2,020,920 193,5601,024 227 27,258 0 Toluene 7,304.7 782,027 29,107 811,133 818,437 35,971439 0 7,303 0 M-xylene 54.9 9,352 157 9,509 9,565 207 5 0 55 0 P-oxylene41.5 7,618 112 7,729 7,771 149 4 0 42 0 P-xylene 58.9 9,860 170 10,02910,089 223 6 0 59 0 Ethylbz 72.5 11,603 215 11,818 11,892 282 7 0 73 0Styrene 34.0 6,293 90 6,383 6,417 121 4 0 34 0 Water 4,266.7 11,299 711,306 15,573 4,268 6 1,244 1 3,022 PD 8,127.7 715 26 742 966 250 0 2223 0 Isoprene 7,499.2 622 29 651 808 185 0 154 3 0 Hexadien 4,147.585,000 11,044 96,044 98,172 13,124 48 657 1,472 0 Hexene 0.0 56,1077,337 63,444 65,512 9,374 31 1,130 939 0 Pentane 0.0 18,965 419 19,38425,388 6,412 11 5,967 37 0

The invention claimed is:
 1. A process for the hydrogenation ofacetylenes and dienes in a stream containing hydrogen, methane, C₂-C₆olefins and paraffins, C₂-C₆ acetylenes and dienes, benzene, toluene,xylenes, and other C₆+ components comprising passing said stream over ahydrogenation catalyst contained in a downflow boiling point reactorwherein the reactor is operated at the boiling point of the mixture inthe reactor and the heat of reaction is absorbed by the boiling liquidand where a portion of the acetylenes and dienes are converted to theircorresponding olefins and paraffins of the same carbon number.
 2. Theprocess according to claim 1 wherein the liquid and vapor in theeffluent from said downflow boiling point reactor is separated and aportion of the liquid is recycled back to the top of said downflowboiling point reactor.
 3. The process according to claim 2 wherein theamount of the liquid being recycled is maintained sufficient to ensurethat the catalyst is fully wetted at all positions within said downflowboiling point reactor.
 4. The process according to claim 2 wherein thevapor in said effluent is fed to a C₅/C₆ splitter where C₅ and lightermaterial are taken as overheads and C₆ and heavier material is taken asbottoms.
 5. The process according to claim 4 wherein a side draw istaken from said C₅/C₆ splitter and fed to the top of said downflowboiling point reactor.
 6. The process according to claim 5 wherein thesteady state composition of the liquid flowing in said downflow boilingpoint reactor is controlled by the location of the draw point of saidside draw along the height of said C₅/C₆ splitter.
 7. The processaccording to claim 6 wherein said side draw is taken from the bottomstage of said C₅/C₆ splitter.
 8. A process for the hydrogenation ofacetylenes and dienes in a stream containing hydrogen, methane, C₂-C₆olefins and paraffins, C₂-C₆ acetylenes and dienes, benzene, toluene,xylenes, and other C₆+ components comprising the steps of: (a) passingsaid stream over a hydrogenation catalyst contained in a downflowboiling point reactor wherein the reactor is operated at the boilingpoint of the mixture in the reactor and the heat of reaction is absorbedby the boiling liquid and where a portion of the acetylenes and dienesare converted to their corresponding olefins and paraffins of the samecarbon number; (b) separating the liquid and vapor contained in theeffluent from said downflow boiling point reactor; (c) returning aportion of the separated liquid to the top of said downflow boilingpoint reactor; (d) maintaining the amount of the liquid being returnedto ensure that the catalyst is fully wetted at all positions within saiddownflow boiling point reactor; (e) feeding the vapor in said effluentto a C₅/C₆ splitter where C₅ and lighter material are taken as overheadsand C₆ and heavier material are taken as bottoms; (f) taking a side drawfrom said C₅/C₆ splitter and feeding said side draw to the top of saiddownflow boiling point reactor; and (g) controlling the steady statecomposition of the liquid flowing in said downflow boiling point reactorby selecting the position of said side draw along the height of saidC₅/C₆ splitter.
 9. The process according to claim 8 wherein said sidedraw is taken from the bottom stage of said C₅/C₆ splitter.